EP1431722B1 - Differential pressure sensor for measuring the liquid level in a container - Google Patents

Description

The invention relates to a differential pressure sensor for measuring the height of the liquid in a tank of a metering system, wherein in the tank a liquid, in particular urea-water solution is filled, and a use of this differential pressure sensor for determining the density of a liquid.

State of the art

It is known from the prior art to determine the level of a liquid in a tank by means of pressure sensors. In particular, by determining the pressure difference between two measuring points, the height of a water column can be measured.

Even in filled with urea-water solutions tanks, it is known to determine the level of the tanks. Thus, the level of a tank filled with urea-water solution, using the so-called echo sounder method determined. An ultrasonic pulse emanating from the depth sounder is emitted from the bottom of the tank and reflected at the surface of the liquid. From the known duration of the sound waves in the liquid, the level is determined. This method has several disadvantages. First, this method is costly. An echosounder that has materials that are resistant to urea solution is expensive to manufacture. On the other hand, the sonar can not be operated with batteries, especially at high filling heights because of the resulting high power consumption. This causes increased energy consumption and thus influences, for example, the energy consumption of an internal combustion engine. In addition, this method can only be used in tanks where the ultrasonic waves are not hindered or reflected by obstacles in their propagation. This considerably reduces the possible designs of the tank.

Another method of level measurement in liquid tanks is the pressure measurement with capacitive sensors. When measuring pressure with capacitive sensors, the change in the distance of the two capacitor plates depending on the pressure is used. Also problematic here are the relatively high costs. The use of capacitive sensors in a urea solution causes high costs because special urea-resistant capacitor plates are required. Furthermore, the sensor must be resistant to the freezing of the urea-water solution.

From the DE 197 55 056 A1 It is known to provide a reference pressure by filling a measuring tube up to a certain height.

A disadvantage of this known prior art; that a reliable measurement of the level is possible only with additional provision of a liquid column of known height.

The DE 3929344 describes a level indicator for fuel tanks with a differential pressure measuring probe.

The DE 4112559 shows a liquid container in which a level measurement using a working according to the Venturi principle jet pump takes place.

Advantages of the invention

The subject-matter of the independent claim, on the other hand, has the advantage of a simple and robust construction, which is particularly important with regard to the solidification of a liquid urea-water solution at -11 degrees Celsius. The differential pressure sensor according to the invention can be used in particular for the continuous measurement of the height of the liquid in a tank for a dosing system for aggressive liquids and can be used in different tank shapes. By the measure to provide a differential pressure sensor with at least a first, non-deformable, liquid-resistant pressure transmitter, in particular for measuring the pressure at the bottom of the tank, it is possible in a simple and cost-effective manner, the amount of aggressive liquids in a tank for a dosing to determine, wherein the shape of the tank can be arbitrary. It is advantageous that the differential pressure sensor has at least a first, as easily deformable by the hydrostatic pressure of the liquid pressure transducer, in particular a membrane. By completing the pressure transducer overcoming the height difference of the tank with a pressure transducer, the problem of random signal change resulting in an open conduit is avoided. The differential pressure sensor forms a closed system, wherein in the system, the pressure-transmitting Medium is located. An open line would have the disadvantage that it would constitute a liquid trap. In this can be conveyed by sloshing of the tank contents or condensation from the gas phase liquid. Conversely, liquid in the line can be transported out of the line by jolts. This results in random signal changes that interfere with a level measurement.

The differential pressure sensor advantageously has at least one liquid-resistant, second pressure transmitter that is not deformable by the hydrostatic pressure of the fluid. This second pressure transmitter is completed in the same way as the first pressure transmitter with a pressure transducer. With two pressure transducers, the measuring unit of the differential pressure sensor can be positioned at any height. If two pressure transducers are used, this has the advantage that the measuring unit of the differential pressure sensor used does not have to be resistant to the liquid present in the tank, but only to the medium present in the pressure transducers. This can be particularly advantageous if the liquid present in the tank can freeze.

Further advantages result from the further features mentioned in the dependent claims and in the description.

The pressure transmitter is particularly resistant to urea-water solution or liquids, which are used as a reducing agent for the reduction of NO _x in exhaust gases of internal combustion engines. The pressure transmitter runs over the entire height of the tank to the floor. Pressure transmitters associated lines can be located inside or outside the tank. The shape of the pressure transmitter can be arbitrary depending on the design of the tank. The pressure transmitter has a material which is not deformable by the hydrostatic pressure of the liquid.

The pressure-tight pressure transmitter can be divided into two chambers by an easily deformable pressure transducer, in particular a flexible membrane, which may consist of rubber, plastic or metal. A chamber is connected to the measuring unit of the differential pressure sensor. If the pressure in the chambers differs, the diaphragm bulges in the direction of the lower pressure. In a further embodiment of the invention, a pressure sensor can be designed as a bellows with a low spring constant. Alternatively, a training as a balloon filled with liquid or a bag made of a tear-resistant film or a tear-resistant fabric would be conceivable.

Conveniently, both the first pressure transmitter, and the second pressure transducer in the form of an elongated hollow profile. In this hollow profile is a pressure-transmitting medium. This can be, for example, air or oil.

Ideally, a pressure transducer, the so-called measuring pressure transducer, arranged at the bottom of the container. A second pressure transducer, the so-called reference pressure transducer, is located in the upper part of the tank near the upper side of the tank. The reference pressure transducer is used to measure the gas pressure above the liquid level in order to ensure a temperature-independent and thus reliable determination of the filling level in the tank.

Due to the expansion of the measuring pressure sensor at the bottom of the tank, a signal is transmitted to the measuring unit of the differential pressure sensor via the pressure-transmitting medium. In order to obtain a signal independent of temperature fluctuations, the pressure-transmitting medium should be able to expand or contract without the pressure within the pressure transmitter or the pressure transducer deviating from the ambient pressure. The pressure-transmitting medium is designed so that pressure fluctuations in the environment do not cause the pressure within the pressure transmitter or the pressure sensor is different from the ambient pressure. The pressure transducers ideally have the lowest possible spring constant. Furthermore, the pressure-transmitting medium has the lowest possible compressibility and the lowest possible coefficient of thermal expansion.

Conveniently, the material of a pressure transmitter and a pressure transducer against the liquid in the tank, in particular against urea, resistant. It is advantageous if the material of a pressure transmitter and a pressure transducer is a urea-resistant thermoplastic or urea-resistant elastomer or metal. Thermoplastics such as IXEF, PA66, PTFE, PPS compositions, PEEK or PVDF prove to be particularly suitable materials which are particularly resistant to urea. But also elastomers, such as BR, CR, HNBR, NR, FPM, EPDM or NBR, as well as metals, such as V4A steel, are suitable materials.

An adjustment of the characteristic curve of the differential pressure sensor is possible if both the measuring pressure transducer and the reference pressure transducer are arranged in the tank so that the liquid in the tank can enclose both pressure transducer when the tank is completely filled. The characteristic curve is adjusted when the tank level rises higher than the reference pressure transducer. Then the signal at the measuring unit of the differential pressure sensor is independent of the filling level of the tank. The adjustment is also possible if, as described above, is dispensed with a second pressure transducer or pressure transducer. In this case, the tank must be filled with liquid so far that the liquid level both the Pressure sensor and the measuring unit of the reference pressure sensor exceeds.

It is of great advantage that the differential pressure sensor can be used in a metering system which is connected to an exhaust gas tract of an internal combustion engine, in particular a vehicle engine.

It is also advantageous that the differential pressure sensor can be used in liquids which serve as reducing agents for reducing the NO _x emissions of the exhaust gas of an internal combustion engine.

In an advantageous embodiment of the invention, the pressure transducer and mounted in the tank pressure transmitter are protected by a mechanical protection, in particular by a sheath. This ensures that the pressure transducer and pressure transmitter is protected against any hurdles of frozen blocks of urea-water solution.

Furthermore, the differential pressure sensor according to the invention can be used to determine the density of a liquid. In the case of the urea dosing system can be easily identified with such use, whether the tank with a liquid other than urea-water solution, for. B. with water or diesel fuel was filled. The density of the pressure-transmitting medium is as equal as possible to that of the vehicle user in the tank to be filled liquid medium. By this measure, possible, caused by tilting or accelerating the vehicle measurement errors are minimized.

In a particularly advantageous embodiment of the differential pressure sensor can be provided, the two pressure transmitter is not perpendicular to each other but to arrange offset in two spatial directions. In a tilted vehicle would result in such an arrangement of the pressure transducer no systematic measurement error (due to low effective measurement height is not systematically too low density or level displayed). The effective measurement height is increased and decreased randomly in a random manner. The evaluation of the signal is carried out by averaging over several measured values, thus resulting in a correct signal.

drawing

Figur 1

einen Differenzdrucksensor in einem Tank für aggressive Flüssigkeiten eines Dosiersystems,

Figur 2

einen Differenzdrucksensor in einem Tank für aggressive Flüssigkeiten eines Dosiersystems, in einem alternativen Ausführungsbeispiel, und

Figur 3

einen Differenzdrucksensor in einem Flüssigkeitstank, bei dem der zweite Druckaufnehmer zur Messung eines Gasdruckes eingesetzt wird.

Further details and advantages of the differential pressure sensor will become apparent from the following description and the accompanying drawings in which preferred differential pressure sensors are shown with the necessary details. Show it

FIG. 1

a differential pressure sensor in a tank for aggressive liquids of a dosing system,

FIG. 2

a differential pressure sensor in a tank for aggressive liquids of a metering system, in an alternative embodiment, and

FIG. 3

a differential pressure sensor in a liquid tank, in which the second pressure sensor is used to measure a gas pressure.

Advantageous embodiments

FIG. 1 shows a metering system 4 with a metering device 9 and a tank 3 for aggressive liquids 2. The liquid 2 passes from the tank 3 via a line 10 to the metering device 9. The metering system 4 is used to liquid, in particular reducing agent, in the exhaust gas of a To convey internal combustion engine, thereby the NO _x emission of the exhaust gas can be reduced.

The differential pressure sensor 1 indicates the level of the liquid 2 in the tank 3 of the dosing system 4. The differential pressure sensor 1 has a first pressure transmitter 5, which runs from the above the tank 3 measuring unit 11 of the differential pressure sensor 1 over the entire height of the tank 3 to the bottom of the tank 3. At the end of the first pressure transmitter 5, a first pressure transducer 7 is arranged. The first pressure transducer 7 has an easily deformable material. This pressure sensor 7 represents the measuring pressure transducer. The second pressure transducer 12 is located in the upper region of the tank 3 and represents the reference pressure transducer. The differential pressure sensor 1 forms a closed system. Pressure transmitter 5, 6 and pressure transducer 7, 12 surround a pressure-transmitting medium 8 tight. The pressure-transmitting medium 8 transmits pressure changes to the first pressure transducer 7 to the measuring unit 11 on. Depending on the hydrostatic pressure at the pressure transducer 7, the fill level of the liquid 2 in the tank 3 can be determined as follows.

mit $${\mathrm{p}}_{\mathrm{meß}}\mathrm{=}{\mathrm{\rho }}_{\mathrm{f}\mathrm{1}}\mathrm{g}{\mathrm{h}}_{\mathrm{f}\mathrm{1}}\mathrm{-}{\mathrm{\rho }}_{\mathrm{Üb}}{\mathrm{gh}}_{\mathrm{ref}}\mathrm{+}{\mathrm{p}}_{\mathrm{Umgebung}}$$

und $${\mathrm{p}}_{\mathrm{ref}}\mathrm{=}\mathrm{-}{\mathrm{\rho }}_{\mathrm{Üb}}{\mathrm{gh}}_{\mathrm{ref}}\mathrm{+}{\mathrm{p}}_{\mathrm{Umgebung}}\mathrm{falls}{\mathrm{h}}_{\mathrm{f}\mathrm{1}}\mathrm{<}{\mathrm{h}}_{\mathrm{ges}}\mathrm{-}{\mathrm{h}}_{\mathrm{ref}}$$

$${\mathrm{p}}_{\mathrm{ref}}\mathrm{=}{\mathrm{\rho }}_{\mathrm{f}\mathrm{1}}\mathrm{g}\left({\mathrm{h}}_{\mathrm{f}\mathrm{1}}\mathrm{-}{\mathrm{h}}_{\mathrm{ges}}\mathrm{+}{\mathrm{h}}_{\mathrm{ref}}\right)\mathrm{-}{\mathrm{\rho }}_{\mathrm{Üb}}{\mathrm{gh}}_{\mathrm{ref}}\mathrm{+}{\mathrm{p}}_{\mathrm{Umgebung}}\mathrm{falls}{\mathrm{h}}_{\mathrm{f}\mathrm{1}}\mathrm{<}{\mathrm{h}}_{\mathrm{ges}}\mathrm{-}{\mathrm{h}}_{\mathrm{ref}}$$

wobei:

• h = Höhe,
• g = Erdbeschleunigung,
• ρ = Dichte

mit den Indizes:

• meß: Messseite,
• ref: Referenzseite,
• ges: Gesamthöhe des Tanks
• Üb: Druckübertragendes Medium
• fl: Flüssigkeit im Tank

The differential pressure Δp measured by the measuring unit 11 is calculated as follows: $$\mathrm{Ap}\mathrm{=}{\mathrm{p}}_{\mathrm{meß}}\mathrm{-}{\mathrm{p}}_{\mathrm{ref}}$$

With $${\mathrm{p}}_{\mathrm{meß}}\mathrm{=}{\mathrm{\rho }}_{\mathrm{f}\mathrm{1}}\mathrm{G}{\mathrm{H}}_{\mathrm{f}\mathrm{1}}\mathrm{-}{\mathrm{\rho }}_{\mathrm{Ov}}{\mathrm{gh}}_{\mathrm{ref}}\mathrm{+}{\mathrm{p}}_{\mathrm{Surroundings}}$$

and $${\mathrm{p}}_{\mathrm{ref}}\mathrm{=}\mathrm{-}{\mathrm{\rho }}_{\mathrm{Ov}}{\mathrm{gh}}_{\mathrm{ref}}\mathrm{+}{\mathrm{p}}_{\mathrm{Surroundings}}\mathrm{if}{\mathrm{H}}_{\mathrm{f}\mathrm{1}}\mathrm{<}{\mathrm{H}}_{\mathrm{ges}}\mathrm{-}{\mathrm{H}}_{\mathrm{ref}}$$

$${\mathrm{p}}_{\mathrm{ref}}\mathrm{=}{\mathrm{\rho }}_{\mathrm{f}\mathrm{1}}\mathrm{G}\left({\mathrm{H}}_{\mathrm{f}\mathrm{1}}\mathrm{-}{\mathrm{H}}_{\mathrm{ges}}\mathrm{+}{\mathrm{H}}_{\mathrm{ref}}\right)\mathrm{-}{\mathrm{\rho }}_{\mathrm{Ov}}{\mathrm{gh}}_{\mathrm{ref}}\mathrm{+}{\mathrm{p}}_{\mathrm{Surroundings}}\mathrm{if}{\mathrm{H}}_{\mathrm{f}\mathrm{1}}\mathrm{<}{\mathrm{H}}_{\mathrm{ges}}\mathrm{-}{\mathrm{H}}_{\mathrm{ref}}$$

in which:

• h = height,
• g = gravitational acceleration,
• ρ = density

with the indices:

• measure: measurement page,
• ref: Reference page,
• tot: total height of the tank
• Üb: pressure transmitting medium
• fl: liquid in the tank

If the liquid level falls below the second (reference) pressure transducer 12, that h _fl <h _ges - h _ref, then for the measured differential pressure Ap: $$\mathrm{Ap}\mathrm{=}{\mathrm{\rho }}_{\mathrm{f}\mathrm{1}}{\mathrm{gh}}_{\mathrm{f}\mathrm{1}}\mathrm{-}{\mathrm{\rho }}_{\mathrm{Ov}}\mathrm{G}\left({\mathrm{H}}_{\mathrm{ges}}\mathrm{-}{\mathrm{H}}_{\mathrm{ref}}\right)$$

The measured differential pressure Δp is consequently linearly dependent on the filling level h _{fl of} the liquid 2 in the tank 3. Since the other variables are known in the above equation, this results in the respective filling level height of the liquid.

In the event that the filling level of the liquid is above the second (reference) pressure transducer 12, Δp results for the measured differential pressure: $$\mathrm{Ap}\mathrm{=}{\mathrm{\rho }}_{\mathrm{Ov}}\left({\mathrm{H}}_{\mathrm{ges}}\mathrm{-}{\mathrm{H}}_{\mathrm{ref}}\right)\mathrm{+}{\mathrm{\rho }}_{\mathrm{f}\mathrm{1}}\left({\mathrm{H}}_{\mathrm{ges}}\mathrm{-}{\mathrm{H}}_{\mathrm{ref}}\right)$$

It can be seen that the measured differential pressure no longer depends on the actual filling level h _{fl of} the liquid 2 in this case.

An adjustment of the characteristic curve of the differential pressure sensor 1 can take place when the tank level rises higher than the reference pressure transducer. The signal then becomes independent of the filling level.

A full tank can be detected by a control unit when, on the one hand, a sudden increase in the tank level is detected and, on the other hand, an amount of fluid is withdrawn from the tank without a change in the differential pressure signal being observed. In the case of a diesel fuel tank, for example, the removal of the liquid can be followed by monitoring the amount of fuel supplied to the engine in the control unit.

As a further criterion for a full tank, it can be used that during driving operation no or only slight fluctuations of the differential pressure signal measured by the measuring unit 11 are observed.

The in FIG. 2 shown differential pressure sensor l 'of the dosing system 4 has largely the same components as the differential pressure sensor 1 FIG. 1 on. At the first Ducküberträger 5 a branch 20 is additionally provided, which connects the first pressure transducer 5 and the first pressure transducer 7 with a measuring unit for density measurement 21. The measuring unit for density measurement 21 has a third pressure transmitter 25. At the end of the third pressure transmitter 25, a third pressure transducer 27 is arranged. The third pressure transducer 27, which for measuring the density of the Liquid 2 is formed, via the measuring unit for density measurement 21 with the first pressure transducer 7, which is arranged in the lower region of the tank 3, united. With a sufficiently accurate drift-free measuring unit for density measurement 21, the density measurement at full tank 3 can be done without the need for adjustment.

The mathematical description is given according to the above formulas for the case h _fl> h _ges - h _ref. On the second pressure transducer 6 and the measuring unit of the differential pressure sensor 11 may be omitted if the device shown is to be used exclusively for density measurement. In the case of the urea dosing system 4 according to the invention, the in FIG. 2 shown embodiment are used to provide evidence that the tank 3 has been filled with a liquid other than urea-water solution. It is conceivable, for example, that water or diesel fuel could get into the tank 3. In an advantageous variant, the density of the pressure-transmitting medium 8 is as equal as possible to the density of the liquid 2 to be filled into the tank 3. Thus, measuring errors caused by slanting or acceleration of the vehicle during driving operation are minimized.

FIG. 3 shows a connected to a tank and a metering differential pressure sensor assembly. At the bottom of the tank is located, for example, centrally disposed, a first pressure receiving member 77, and at a height H above the bottom of the tank, a second pressure receiving member 78 is arranged. The tank is dimensioned so that its maximum filling height, at which the tank is defined as "voli", is smaller than the height H, so that the second pressure-receiving element is above the liquid level even when the tank is full. The second pressure receiving element 78 is connected via a signal line 79 with an evaluation circuit 80, as well as the first pressure receiving element 77 is connected to the circuit 80. At the output of the circuit 81, a pressure signal line is connected, which is connected to a signal input of a control device, not shown. The tank 3 has a filler neck 82 in the upper area. Near the bottom, a line 88 is connected, which leads via a feed pump 84 to an electrically controllable via the control unit metering valve 87. About the metering valve, the liquid contained in the tank can be introduced, for example in the exhaust system of a motor vehicle. Between the metering pump and metering valve branches off a return line 83 from the line 88, which returns via a line pressure sensor 85 and a pressure regulator 86 to the upper portion of the tank.

The control unit not shown controls the function of the electrically controllable feed pump, the electrically controllable metering valve and the electrically controllable pressure regulator in response to the signal of the line pressure sensor 85 and other data not shown here such as the speed, the load of the motor vehicle engine and possibly other signals from In the exhaust system arranged exhaust gas sensors. The signal applied to the pressure signal line 81 serves the control unit for detecting the fill level in the tank, so that a fuel gauge can be controlled or that a warning in the dashboard of a truck can be issued in good time before reaching a certain lower level that, for example, a 32, 5% urea-water solution must be topped up to continue to ensure exhaust gas after-treatment that complies with legal standards.

The level height and the pressure signal of the pressure-receiving element 77 are stored here depending on the tank design specific and in a characteristic curve in the control unit. The detection of the level is preferably carried out when the vehicle is stationary when refueling or idling to exclude errors by sloshing reducing agent while driving. At elevated ambient temperature up to 60 degrees Celsius, the increased vapor pressure of the reducing agent can set an increased pressure in the tank, which would falsify the fill level signal in the line 81 if it were based solely on a signal of the first pressure receiving element 77. This is compensated by the fact that the gas pressure in the upper region of the tank is measured by the second pressure receiving element 78 and forwarded via the line 79 to the evaluation circuit 80, so that with appropriate design of the evaluation circuit 80, a gas pressure compensated signal is applied to the line 81, which according to the above mentioned in the control unit stored map is a clear and temperature-independent measure of the filling level. In a simple arrangement, the difference between the signal of the first pressure-receiving element and the signal of the second pressure-receiving element is formed in the evaluation circuit 80. To avoid draining the tank, the fill level of the tank is updated while the vehicle is driving using an in-house calculation of the reducing agent consumption. The consumption is determined here from the control data of the metering or of the metering valve (metering pressure, temperature, activation duration, drive frequency, etc.) and actually or at certain intervals from the last actual subtracted measured level. The difference then gives the new current level. When the vehicle is stationary, a further adjustment with regard to the fill level can additionally be carried out via a pressure measurement in the tank. This increases the safety in determining the level of reducing agent to meet the statutory on-board diagnostic guidelines.

In a modification, the evaluation circuit 80 can be replaced by a special embodiment of the pressure-receiving element 77, in which the signal line 79 is replaced by an air-tight pipeline, which leads the pressure prevailing in the upper region of the tank gas pressure to the pressure receiving element 77. The second pressure-receiving element 78 is replaced in this case by a pressure-receiving means, which is formed by the projecting into the upper part of the tank open end of the pipeline. The pressure-receiving element 77 then generates, with a corresponding mechanical construction, an electrical signal which corresponds to the pressure difference between the liquid pressure at the bottom of the tank and the gas pressure in the upper region of the tank.

In general, pressure-absorbing means may for example be understood to mean an open end of a pressure transmitter, a deformable part (pressure sensor) for closing the open end of a pressure transmitter or a pressure-receiving element which supplies an electrical signal which correlates with the applied pressure or differential pressure.

Claims (14)

1. Differential pressure sensor for measuring the liquid level of a liquid, in particular of a urea/water solution, which has been filled into a container of a metering system, wherein a first pressure-sensing means (7, 77) for measuring a pressure at the bottom of the container and a second pressure-sensing means (12, 78) for measuring a reference pressure in an upper area of the container are provided, wherein the second pressure-sensing means is arranged at such a mounting height (h_ges - h_ref, H) in the container that the second pressure-sensing means (12, 78) is surrounded by liquid (2) at most when a liquid level is reached at which the container is defined as full, wherein the differential pressure sensor (1) has at least a first, nondeformable, liquid-resistant pressure transmitter (5), in particular for measuring the pressure at the bottom of the container (3), wherein the differential pressure sensor (1) has at least one nondeformable, liquid-resistant, second pressure transmitter (6), in particular for measuring a reference pressure in the upper area of the container (3), characterized in that the pressure-sensing means (7, 12) are arranged at the open end of the respective pressure transmitter (5, 6), and in that the differential pressure sensor (1) forms an enclosed system, wherein a pressure-transmitting medium (8) is surrounded by the pressure transmitters (5, 6) and the pressure-sensing means (7, 12), wherein the pressure-sensing means (7, 12) have a deformable part.
2. Differential pressure sensor according to Claim 1, characterized in that the pressure transmitter or transmitters (5, 6) are in the form of an elongate hollow profile.
3. Differential pressure sensor according to Claim 1 or 2, characterized in that the deformable part is a diaphragm, a folding bellows or a balloon.
4. Differential pressure sensor according to Claim 1, characterized in that the material of the pressure transmitters (5, 6) and of the pressure-sensing means (7, 12) is resistant to the liquid in the container (3), in particular to a urea/water solution.
5. Differential pressure sensor according to Claim 4, characterized in that the material of the pressure transmitters (5, 6) and of the pressure-sensing means (7, 12) is a urea-resistant thermoplastic or urea-resistant elastomer or metal.
6. Differential pressure sensor according to one of the preceding claims, characterized in that the differential pressure sensor (1) is configured for use in a metering system (4) which is connected to an exhaust section of an internal combustion engine, in particular of a vehicle engine.
7. Differential pressure sensor according to one of the preceding claims, characterized in that the differential pressure sensor (1) is configured for use in liquids (2) which serve as reducing agents for reducing the NO_x emissions of the exhaust gas in the internal combustion engine.
8. Differential pressure sensor according to one of the preceding claims, characterized in that said sensor is configured for adjusting a characteristic curve of the differential pressure sensor (1) when the container is full.
9. Differential pressure sensor according to one of the preceding claims, characterized in that a control unit is configured for detecting a (sudden) increase in the level in the container.
10. Differential pressure sensor according to one of the preceding claims, characterized in that the pressure transmitter (5, 6) and/or the pressure-sensing means are protected by mechanical protection, in particular by an encapsulation means.
11. Differential pressure sensor according to one of the preceding claims, characterized in that both pressure-sensing means (7, 12) are arranged in the container in such a way that the liquid surrounds both pressure-sensing means (7, 12) when the container is full.
12. Differential pressure sensor according to one of Claims 1 to 10, characterized in that the mounting height of the second pressure-sensing means is selected such that the second pressure-sensing means is not surrounded by liquid even when the container is defined as full.
13. Differential pressure sensor according to Claim 12, characterized in that the second pressure-sensing means detects a gas pressure above the surface of the liquid.
14. Use of a differential pressure sensor according to one of Claims 1 to 13 for determining the density of a liquid.

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